Skycages

The skycages are cells in a prison-like area at the top of the Tower. They are prominent from the outside, seen hanging from beams. Dialogue between Siris and the guardian of the prison reveals that the prisoners are "traitors to the Deathless Houses and the Great Pact".

Go to the hall that leads to both Thane and MX-Goliath. Take the door leading to Thane, and you will reach a small room with two doors. The door on the left leads to the skycages, and requires a large key to unlock. Once inside, you must defeat either the Headsman or the Butcher. After the battle, tap the location on the left to approach the three cells. Note: Thane must have been defeated on a previous Rebirth in the same game for the area to be accessible.

Three skycages can be accessed:

The skycage on the left requires Nobel to be opened. Inside is the Petrified Noc, who immediately attacks Siris after released. He is also the only prisoner in the skycages who does not talk to Siris.

The skycage in the center requires the Infinity Blade to be opened. Inside is the Iron Hunter, who attacks Siris, thinking he is an accomplice of his captors. When the Iron Hunter is nearly defeated, he yields. Siris can then decide whether to spare or finish him. If you spare the Iron Hunter, he gifts the player with Torren's Legacy in the next Rebirth. If you don't spare the Iron Hunter you will get the Hunter Mail.

The skycage on the right requires The Widow Shriek to be opened. Inside is Xyloto, who battles Siris. He does not seem to tolerate Siris' generosity and appears to hate him instead.

A game breaking bug occurs if a player spares the Iron Hunter twice. The only ways to fix this bug are to restart to Rebirth 1 or follow the instructions in this post. This bug is fixed in 1.3.1 update. The second time you meet Iron Hunter, you will be rewarded of some gold instead of another Torren's Legacy (though the cutscene stays the same).

Equip which weapon you need. Then when you go to press the button quickly press the menu button at the top of the screen. If you did this quickly enough you will be able to equip another more powerful weapon.

There is a bug when fighting the prisonner on the left.
The bug appears when you happen to get the stabbing event after landing a perfect parade. If you press the Magic Icon at the right time you can infinitely stab him until he dies.